Electrochemical devices using nonaqueous electrolytes, represented by a lithium secondary battery and a supercapacitor, have been used widely as power sources for portable equipment such as mobile phones and notebook-sized personal computers because of the characteristic of high energy density. With improvement in the performance of the portable equipment, the capacity of the electrochemical devices tends to become higher, and thus securing safety has become important.
In a current-technology lithium secondary battery, a polyolefin-based porous film having a thickness of 20 to 30 μm is used as a separator to be interposed between a positive electrode and a negative electrode, for example. For the material of the separator, polyethylene (PE) having a low melting point may be used for securing a so-called shutdown effect, namely, melting a resin as an ingredient of the separator at or below a thermal runaway (abnormal heating) temperature of the battery so as to close the pores, thereby increasing the internal impedance of the battery and improving the safety of the battery at the time of a short-circuit or the like.
For the separator, for example, a uniaxially- or biaxially-stretched film is used in order to provide porosity and improve the strength. Since such a separator is provided as a film to exist alone, a certain strength is required in view of workability or the like, which is secured by the above-mentioned stretching. However, since crystallinity of the stretched film is increased, and the shutdown temperature is raised up to approximately the thermal runaway temperature of the battery, the margin for securing the safety of the battery cannot be provided sufficiently.
Moreover, distortion occurs in the film due to the stretching, and thus when exposed to a high temperature, shrinkage will occur due to residual stress. The shrinking temperature is extremely close to the melting point, that is, the shutdown temperature. As a result, in the case of using a polyolefin-based porous film separator, when the temperature of the battery reaches the shutdown temperature during anomalies in charging or the like, the current must be decreased immediately for preventing the battery temperature from rising. If the pores are not closed sufficiently and the current cannot be decreased immediately, the battery temperature will be raised easily to the shrinking temperature of the separator, causing a risk of abnormal heating due to an internal short-circuit.
In order to prevent a short-circuit caused by such thermal shrinkage, methods of using separators of a microporous film or a nonwoven fabric using a heat-resistant resin have been proposed. For example, Patent document 1 discloses a separator using a microporous film of wholly aromatic polyamide, and Patent document 2 discloses a separator using a polyimide porous film. Patent document 3 discloses a separator using a polyamide nonwoven fabric, Patent document 4 discloses a separator including a base of a nonwoven fabric using aramid fibers, Patent document 5 discloses a separator using a polypropylene (PP) nonwoven fabric, and Patent document 6 discloses a separator using a polyester nonwoven fabric.
Though each of the above-mentioned separators made of a heat-resistant resin or heat-resistant fibers has excellent dimensional stability at a high temperature and can be made thinner, it does not have the so-called shutdown characteristic, namely, a characteristic that the pores will be closed at a high temperature, and thus the separator cannot provide sufficient safety at an abnormality, specifically when the battery temperature rises rapidly due to an external short-circuit or an internal short-circuit.
As a method for solving such problems, Patent documents 7 and 8 show separators including a base of a nonwoven fabric, in which thermoplastic polyolefin is contained. Such separators indeed cause no thermal shrinkage at a high temperature, and show the shutdown characteristic with polyolefin melted at its melting point or higher. However, studies of the present inventors have shown that the above-described separators have a problem in securing the reliability of a battery for the following reason. That is, in the case where a positive electrode containing an active material of inorganic oxide particles, which usually are used for a positive electrode of a lithium battery, is used together to provide a battery, for example, due to the flexible polyolefin and the very hard inorganic oxide particles, when the positive electrode and a negative electrode are pressed against each other via the separator, protrusions of the inorganic oxide particles of the positive electrode may penetrate the separator to be in contact with the negative electrode, resulting in the possibility of a short-circuit.
Patent documents 9 and 10 propose methods of forming a separator that contains an inorganic filler in a nonwoven fabric in order to prevent the short-circuit as described above. However, such a separator is not provided with the shutdown function, and thus has a problem in securing safety. Further, in the examples shown in Patent documents 9 and 10, mere granular fine particles are used for the inorganic filler. However, according to studies of the present inventors, when lithium dendrite is formed, the dendrite is likely to penetrate a porous film formed of granular inorganic fine particles, and thus the reliability with respect to an internal short-circuit caused by the dendrite cannot be secured sufficiently.
Further, in the example shown in Patent document 9, a binder for binding the inorganic filler is not used, while an inorganic binder is used in the example shown in Patent document 10. Such separators have no problem when used without being bent. However, when a positive electrode, a negative electrode, and the separator are wound spirally to form a wound body, which generally is used for a lithium battery, the separator formed of the inorganic filler easily cracks, which may cause a short-circuit. Especially, in the case of a rectangular battery using a wound body having a bent portion of a small diameter, the problem of a short-circuit caused by a crack of the separator is notable.
In addition, Patent document 11 shows a separator in which a shutdown layer formed of polyolefin particles is provided on a porous film formed of a nonwoven fabric and an inorganic filler, thereby securing the shutdown function. With this constitution, it is possible to provide the shutdown function while securing heat resistance of the separator. However, since the porous film as a base formed of a nonwoven fabric and an inorganic filler has the same constitution as that shown in Patent document 10, the above-described problems, that is, resistance to an internal short-circuit caused by the dendrite and the reliability with respect to bending, remain to be solved.    Patent document 1: JP H05-335005 A    Patent document 2: JP 2000-306568 A    Patent document 3: JP H09-259856 A    Patent document 4: JP H11-40130 A    Patent document 5: JP 2001-291503 A    Patent document 6: JP 2003-123728 A    Patent document 7: JP S60-136161 A    Patent document 8: JP H05-74436 A    Patent document 9: JP 2003-22843 A    Patent document 10: JP 2005-502177 A    Patent document 11: JP 2005-536858 A
With the foregoing in mind, it is an object of the present invention to provide a separator that can form an electrochemical device with excellent safety at the time of abnormal heating and reliability with respect to an internal short-circuit caused by various causes and a method for producing the same, and an electrochemical device including the separator and a method for producing the same.